Electric power is often provided in the form of alternating currents and voltages. In North America, for example, the electric power may typically be provided at a frequency of 60 Hz. When the electric power interacts with various electronic devices, current and voltage distortions may occur. The distorted power may affect the operation of many electronic devices. In one example, a building's power distribution system may be connected to various electronic devices comprising, for example, computers, electronic ballasts, printers, fax machines, HVAC equipment, and uninterrupted power supplies. These devices, or non-linear loads, may cause the quality of the building's power supply to degrade.
Harmonics cause distortion in sinusoidal power waveforms. These distortions are referred to as “harmonic distortion” and are caused by various factors. By way of background, a harmonic of a wave, in this case a power signal, is an integer multiple of the fundamental frequency. As mentioned above, the fundamental frequency, or frequency at which electric power is generated, in North America is 60 Hz. In European countries, the fundamental frequency may be 50 Hz, while an aircraft system may generate power at a fundamental frequency of 400 Hz. It is appreciated that the fundamental frequency in a power system may vary. The superposition of a harmonic current and a current operating at the fundamental frequency may generate a distorted current and/or voltage signal, also referred to as harmonic distortion.
Typically, harmonic distortion provides undesirable voltage and current waveforms to electronic devices. It is appreciated that many electronic devices are designed for power input in the form of an undistorted sine wave. Therefore, a distorted power input may cause a variety of problems for electronic devices, including overheating, increased power consumption, reduced power factor, malfunction or failure, erratic operation of breakers and relays, and pronounced magnetic fields near transformers and switch gears. As described herein, filtering or dampening the harmonics refers to the reduction of harmonic distortion.
It has been shown that harmonics may be dampened by using various approaches, including without limitation, active and passive filters. Active filters may use additional energy to dampen the harmonics, and in some cases may rely on the principle of wave cancellation. Active filters generally require costly electronic components, which may have relatively short life expectancies. Furthermore, due to limited capability of electronic components, active filters are typically not suitable for larger current loads.
Passive harmonic mitigation generally utilizes the flow of current to trap or pull the harmonics within a device, thereby preventing the propagation of the harmonics throughout the system. By way of background, passive harmonic filters for dampening or filtering harmonics generally comprise reactors and capacitors. A reactor may comprise at least one inductor and may be used to absorb sudden changes in current. Advantages of passive systems over active ones include lower costs, simplified structure, utility across larger current and voltage ranges, and increased robustness.
In addition to the above considerations, power factor is also of interest in the field of harmonic filtration or mitigation devices. It is appreciated that the power factor is the ratio between the power used (i.e. Active or Real power) and power supplied (i.e. Apparent power) to a device. In ideal circumstances, the power factor would be 1.
An example of a passive filter is provided to U.S. Pat. No. 6,127,743 (the '743 patent) which discloses a harmonic mitigation device comprising two oppositely wound inductors arranged in series. These oppositely wound inductors are further electrically connected in series between an AC source and non-linear load. A third inductor is connected in parallel between the junction of the two series inductors. A capacitor is connected in series to the third inductor. The '743 patent teaches that the inductors are stacked vertically along the length of a common reactor core.
In U.S. Pat. No. 6,844,794 (the '794 patent), a harmonic filter is described using two inductors. The '794 patent shows that one of the inductors that is electrically connected in series to the non-linear load may be removed, while mitigating the effects of the harmonics. In the '794 patent, an embodiment is described with two separate reactors.
There exists a need for a device for dampening harmonics that overcomes at least one of the drawbacks associated with the prior devices.